So why does this sentence ''Rosa craint que nous n'allions en vacances.'' mean that Rosa fears she will not go on vacation?

Question

The wrong answer on the quiz leads to the lesson on "ne expletif" that states that the ne does not have any negative meaning. So that would make it seem that Rosa is fearing that we may go on vacation. But the answer is that Rosa is fearing that we will NOT go on vacation. Can you please break this sentence down and explain how the "ne expletif" fits into this?

Answer

Hi Yoel,
In a negation you ALWAYS need ne + another negating particle like pas, aucun, plus, jamais, etc. It isn't a negation without the other part. A "ne" alone does not a negation make.
This leaves a single "ne" open for other uses within a sentence, and the French language has found one for it: a single "ne" is used to emphasise the contextual relationship between a main clause and a subordinate clause. Particularly when the verb in the main clause carries a negative meaning such as "craindre".
Therefore you need to disreagard the "ne" in the sentence, it is a ne explétif and not a negation. Rosa is afraid that we might go on vacation, not the other way around.
-- Chris (not a native speaker).